Randomly copolymerized polypropylene is widely used for food package. However, its soluble fraction (the solubles) will migrate to the package surface and contaminate the food. This project would focus on evaluating the relationship between the macromolecular structure and the performance of transparent polypropylene with low soluble contents. This will involve the characterization of the soluble fraction, the intermolecular chemical composition distribution in as-polymerized samples and its effect on the solubles, and the characterization of crystallization behavior and the morphology. Factors influencing the soluble fraction would be identified for reaching a balance of low soluble contents and high end-use properties.
The clarified polypropylene (PP) is used more and more widely in food-contacting package and medical articles. US FDA regulations §177.1520 require n-hexane extractable content (at 50℃) to be less than 2.6wt% for olefin copolymer holding food during cooking and less than 5.5wt% for general food packaging applications. China GB9693 requires less than 2wt% of n-hexane extractable content (at b.p. of n-hexane) of polypropylene resin as food packaging material. For a quite long time, propylene/ethylene random copolymers have been used as base resin for clarified PP. However, traditional random copolymers have relatively high n-hexane extracts which possibly exceed the limit for food-contacting application.
Several kinds of clarified PP having low soluble fraction contents have been developed in recent years. Random PP copolymers made by metallocene catalysts have much lower solubles contents, compared with traditional random PP copolymers, because of the uniform co-monomer distribution of the former. They have not been widely used in the transparent package field due to the higher cost of the metallocene catalysts. SINOPEC has commercialized propylene/1-butene random copolymers having very low solubles contents (also called G-resin) by using its high isotacticity Ziegler-Natta catalyst. But the relationship between the macromolecular structure and the performance of the materials remains unclear. For example, little is known about the intermolecular comonomer distribution (chemical composition distribution) and the composition of the solubles. How do the copolymers achieve the good balance among low soluble contents, high transparence and high toughness？What are the crystallization behavior and the effect of the processing conditions on the morphology and the end use properties?
As a feasibility study two preliminary reports have been presented. One is on the structure characterization of the PP G-resin samples, including some preliminary results about the molecular weight distribution and the composition of the solubles. The other is on the investigation of crystallization behavior by means of flash DSC.
page last updated 14 March 2017